In numerous industrial situations, personnel come in physical contact with electrical equipment under conditions highly conducive to electrical shock due to ground faults. Handling of the electrical equipment under any conditions where personnel can become grounded, such as standing on a wet floor, obviously poses the potential for electrical shock. In many cases, industrial electrical equipment is operated at voltage and current levels which are too high for conventional ground fault circuit interrupting (GFCI) devices to accommodate. Thus the highly effective personnel shock protection afforded by conventional GFCI devices utilized in low voltage circuits, e.g., 120 and 240 VAC, is not presently available for circuits operating at higher voltages.
To date, ground fault protection for industrial electrical power circuits has been limited to equipment protection by virtue of the fact that the power circuit is interrupted to clear a ground fault for ground fault currents in the ampere range, much too high for personnel protection considering the fact that conventional GFCI devices must respond to ground fault currents as low as 5 milliamperes. A principle reason for this is that the typical ground fault sensor, e.g., a zero sequence transformer, utilized in industrial power circuits is not sufficiently precise to detect a 5 milliampere differential in the currents flowing to the load and returning to the source as an indirect indication of a 5 milliampere ground fault current. An alternative approach to industrial power circuit ground fault protection is to directly monitor ground fault current utilizing a current transformer coupled with the ground strap solidly connecting the source to ground and through which ground fault current must flow in returning to the source. The current transformer activates a ground fault relay operating to initiate interruption of the industrial power circuit, clearing the ground fault. Again the combination of the ground strap current transformer and ground fault relay is not sufficiently current sensitive to afford effective personnel protection.
It is accordingly an object of the present invention to provide a ground fault protection system for power circuits operating at elevated voltages and currents which affords equipment protection and is also sufficiently sensitive to afford effective personnel protection.
A further object of the present invention is to provide a ground fault protection system of the above character which utilizes as its principle operating component a conventional ground fault circuit interrupting (GFCI) device of the type currently being mass produced for utilization in low voltage, residential type circuits to protect personnel from the hazards of electrical shock due to ground faults.
An additional object of the present invention is to provide a ground fault protection system of the above character which is inexpensive to manufacture, reliable in operation, and convenient to implement.
Other objects of the invention will in part be obvious and in part appear hereinafter.